In hydraulic systems wherein fluids such as oils or the like are passed through a filter, it is desirable to provide an external mechanism for indicating when the filter element has become clogged and requires replacement. Since the pressure drop across a filter increases in proportion to the accumulation of dirt therein, a suitable indication may be obtained by utilizing a differential pressure device set to be actuated when the pressure drop in the filter reaches a predetermined value. There are several known prior art devices which provide external means for indicating clogging of the filter element and which compensate for low temperature operation so that a false clogging signal is not indicated at low fluid temperatures.
One such design is shown in U.S. Pat. No. 2,942,572 to David Pall issued June 28, 1960. In this device, a first magnetic element is arranged to attract a second magnetic element as long as the two elements are separated by less than a predetermined distance. A biasing mechanism propels the second magnetic element to an indicating position whenever the predetermined distance is exceeded. The first magnetic element is movable reciprocally with a piston responsive to changes in pressure, and is normally biased towards the second magnetic element by a predetermined force. The second magnetic element is also movable reciprocally with a piston and while retained towards the first magnetic element by magnetic attraction when close enough to it, it is normally biased in a direction away from the first magnetic element. The biasing force is capable of overcoming the force of magnetic attraction whenever the first and second magnetic elements are separated by the predetermined distance. The magnitude of the force of the magnetic attraction relative to the spring biasing force determines the predetermined pressure differential at which the device is actuated. In order to prevent actuation at low temperatures, a bimetallic strip is positioned to prevent motion of the second magnetic element when the temperature is below a predetermined value. The bimetallic element is comprised of two arcuate strip portions which are joined by a weld and arranged inwardly with decreasing temperatures. The element contracts inwardly so that the inner strip extends over a flange, thus preventing actuation of the pressure indicator. Thus, the bimetallic elements are subjected to severe stresses at low temperatures. This leads to the eventual fatiguing of the bimetallic elements and failure of the pressure indicator. In addition, this device is complex and expensive to make.
Cole, in U.S. Pat. No. 3,117,550, issued Jan. 14, 1964, owned by the assignee of the present application, discloses a temperature sensitive differential pressure indicating device which includes an elastically unstable element. The elastically unstable element consists of a spherically or conically shaped bimetallic disc which is held and sealed along its periphery so that the resultant force acting on the disc from the pressure differential will be in the direction tending to cause the disc to flatten out. When a critical force is reached, however, the disc will suddenly invert with considerable force at a desired pressure and temperature combination and will stay inverted until manually reset or automatically reset when the actuation pressure is released. This device is also complex requiring precise calibration of the disc to respond to the pressure and temperature combination.
Juhasz, in U.S. Pat. No. 3,812,816, discloses a differential pressure indicator with a thermally sensitive element. The device has a pressure sensitive mechanism and an indicator operatively associated with it. The pressure sensitive mechanism moves under the influence of a differential pressure generated in a fluid handling system to allow a suitable indication when a certain pressure differential exists over the parts of the system. The device further has a locking mechanism which insures that the indication will remain until the causes of the pressure difference are corrected. The device also contains a temperature sensitive element in the form of a bimetallic coil strip which at one end is fastened to a piston body with the other end fastened to and importing rotation to a head shank and a head. As the fluid temperature increases, the head is moved in a counterclockwise rotation by the expanding bimetallic strip until notches or cam surfaces are moved into a position behind the locking balls. The balls are then free to withdraw from partial engagement with a slot to enable the piston to freely move in response to any fluid differential pressure, so long as the fluid temperature is maintained. This design is also complex requiring precision machining of the parts in order to form the locking mechanism.
All of the aforesaid prior art designs are complex. In addition, those devices utilizing bimetallic elements try to prevent movement of a piston when the piston is subjected to the high pressure differentials due to cold fluid. This eventually leads to overstressing the parts with the eventual failure of the bimetallic elements.